forensic biology rachel neagle virginia department of forensic science © 2014 virginia department...
TRANSCRIPT
Forensic Biology
Rachel NeagleVirginia Department of Forensic Science
© 2014 Virginia Department of Forensic Science
Outline• Introduction to DNA• Forensic Biology Lab Processing
– Body Fluid Identification– Nuclear DNA Testing
• Analysis– Statistics
• Data Bank• Additional DNA Testing
What is DNA?• Deoxyribonucleic Acid
– Chemical substance found in our bodies that is responsible for determining all aspects of our physical make-up
– “Blueprint of Life”
What is DNA?• Deoxyribonucleic Acid
– The molecular basis of inheritance• ½ from mother and ½ from father
– >99% of DNA is shared between individuals
– Identical siblings share the same DNA
Where is DNA found? Human Cell
Mitochondria• powerhouse of
cell
• Mitochondrial DNA
• high copy number
Nucleus• brain of cell
• Nuclear DNA
• high power of discrimination
www.cstl.nist.gov/bioteh/strbase
Where is DNA found?• Blood (WBCs)• Semen• Saliva• Perspiration/Skin cells• Tissue• Hair• Bone• Teeth• Urine
Structure of DNA
• Chromosomes– Tightly wound,
supercoiled DNA
• 23 pairs of chromosomesh
ttp://www.ncbi.nlm.nih.gov/genome/guide/
Forensic Biology Lab Processing
• Consists of both serological examination and DNA analysis
• Examines evidence for:– The presence/absence of body fluids/tissue (blood, semen,
urine, perspiration, etc)– Determine DNA profile
Biological Evidence
• Body fluids &/or tissue– Blood– Semen– Saliva– Hairs– Bone/teeth– Skin/tissue– Sweat– Urine– Feces
Examples of Touch Evidence
• Keys• Gun grips & triggers• Door & drawer
handles• Light switches• Countertops• Gear shift knobs• Steering wheels• Cigarette lighters
Blood Testing• Chemical Testing
– Phenolphthalin (Kastle-Meyer)– Tetramethylbenzidine– Combined Phenolphthalin Tetramethylbenzidine (PTMB)– Leucomalachite Green (LMG)
• Chemiluminescent Testing– Luminol– Bluestar®
• Immunological Testing– Hematrace®
• Microscopic Exam– Teichman– Takayama
Blood Testing• Chemiluminescent Testing
– Luminol– Bluestar®
• Chemical Testing – Combined Phenolphthalein Tetramethylbenzidine
Phenolphthalein Tetramethylbenzidine
Semen Testing• P30 (prostate specific antigen)
2nd pink line in the Test Area indicates a positive result
Urine, Feces and Saliva Testing• Urine
– Urease Test –breakdown of urea by urease in presence of heat causes basic reaction
• Feces– Edelman’s Test – tests for the presence of urobilin
which is a breakdown product of bilirubin• Saliva
– Phadebas Test- tests for the presence of amylase
These tests require a significant amount of sample
DNA Testing
• DNA Testing– Extraction and purification– DNA quantitation– PCR set-up
• Analysis of results
PCR
• Polymerase Chain Reaction
• Makes millions of copies of a specific region(s) of DNA (AKA amplification)
• Similar to Xeroxing
Automation of DNA Testing
• Advantages:– Increase sample
production/ throughput
– Increase time for examiner to conduct other duties (screen additional cases)
– Decrease risk of contamination
– Increase consistency of results
3100xl Instrument for DNA Analysis
• DNA sample injected automatically onto a capillary column
• DNA separated by electrophoresis
• Data automatically collected
DNA Analysis• Separation of the copied regions based on
size to determine differences and determine a profile of the contributor(s)– Pieces with more repeats, larger, move
slower through gel-like substance– Represented by peaks on a graph
(electropherogram)
• The DNA profile
STRs – Short Tandem Repeats• Non-coding regions of DNA• Alleles assigned numbers based on the number
of 2-7 bp repeated segments of DNA present• Examine 15 different STR loci + gender marker
Nuclear DNA Analysis
AGAT AGATAGAT
AGAT
AGAT
AGAT
AGAT
AGAT
AGAT
AGAT
6
4
DNA Profile = 4,6
One piece inherited from mom, the other from dad
Nuclear DNA Analysis
• Mixture Profiles– DNA profile resulting from more than one
individual contributing to a sample– Three or more peaks (alleles) generally
being detected at multiple areas– Major/minor contributors
Conclusions
• Included (Not eliminated)– Individual cannot be eliminated as a contributor
of a DNA profile or to a DNA mixture profile• Eliminated
– Individual is not the source of the DNA detected• No Conclusions
– Cannot make a determination relative to individual in question
Statistics
• Provides a measure of the weight to the evidence– How common or uncommon is a particular
profile in the population?– How often you would expect the evidence
profile to be found in the population?• (based on allelic frequencies)
Statistics
• Calculated when an individual is included as a possible contributor of the evidentiary DNA profile
• Numerical probability of approximately how rare or how common a given DNA profile is in the population
Factors which limit effectiveness of DNA evidence
• Contamination– Direct contact with another source of DNA– Can occur before, during, or after evidence
collection• Degradation
– High temperatures, humidity– Bacterial growth, natural decaying process
Factors which limit effectiveness of DNA evidence
• Probative value– Evidence left by someone not related to the crime– Evidence obviously left by someone related to
crime• Availability of reference samples
– Mixtures without reference(s) have little value– Complex mixtures without reference(s) typically
have no value
The collection and DNA analysis of blood and buccal samples from convicted felons and Arrestees
DNA profiles obtained from evidence samples are used for comparison to unsolved cases and samples from convicted offenders and arrestees
DNA Data Bank
CODIS
• Software owned and operated by Federal Bureau of Investigation
• Comprised of samples from the federal databank and samples from all 50 states
• Samples include:– Offender– Arrestee– Forensic– Missing persons
NDIS Statistics
• As of June, 2014– Over 11,015,147 offender samples– Over 1,922,415 arrestee samples– Over 565,159 forensic samples– Produced over 250,809 hits
For statistics:http://www.fbi.gov/about-us/lab/biometric-analysis/codis/ndis-statistics
Y-STR Analysis
• Similar to nuclear DNA analysis except focuses on the Y-chromosome
• Paternally inherited• Commercially available amplification kit
for 17 areas on the Y-chromosome
Y-STR Analysis
• Advantages– Extreme female-male ratio cases
• With or without sperm presence– Male-male mixture cases
• Disadvantages– Standards needed for comparison– Not unique to individual-paternally inherited
Where is DNA found? Human Cell
Mitochondria• powerhouse of
cell
• Mitochondrial DNA
• high copy number
Nucleus• brain of cell
• Nuclear DNA
• high power of discrimination
www.cstl.nist.gov/bioteh/strbase
Mitochondrial DNA Analysis
• Mitochondrial DNA– Mitochondria: Cytoplasm– >1000 copies per cell– Maternally inherited
National Institutes of Health Genome.gov
Mitochondrial DNA • Advantages
– Mulitiple copies per cell- >1000 copies– Reference samples more easily obtained
• Maternal relatives– Harder to degrade than nuclear DNA– Found in lower quantity or highly degraded samples
• Hair shafts• Bone
– Body ID, missing persons and mass casualty cases
Photo courtesy of Robert Quick, Norfolk PD
http://commons/wikimedia.org/wiki/Image:CID_post-impact_1.jpg
Mitochondrial DNA
• Disadvantages– Testing is labor intensive and costly– Maternally inherited
• Not unique to an individual• Statistics generally 1 in 100’s to 1 in 1000’s
– Reference standards required because most samples cannot be searched in database
– Not applicable to mixture samples
Acknowledgements
• Virginia DFS co-workers• www.dfs.virginia.gov• www.fbi.gov• www.nih.gov• www.nist.gov